Solar Sheep and Voltaic Veggies: Uniting Solar Power and Agriculture

June 06, 2018 by Benjamin Mow

Sheep grazing in a field of solar panels is becoming an increasingly common sight
as both farmers and solar developers are starting to experiment with co-locating solar
photovoltaic (PV) systems and agriculture. Small-scale, off-grid PV systems located
on farm land was one of the first applications of solar power. The arrangement made sense for low-power agricultural needs in locations where running
distribution lines was too expensive or not possible. More recently, the idea of solar
systems on farm land has expanded to large-scale, grid-connected systems that have
vegetation growing around and under the panels and/or livestock grazing on the same
parcel of land (see Figure 1).

Figure 1. Grazing Sheep Under Ground-Mounted Solar Installation

Grazing sheep can help control vegetation height at grount-mounted solar installation
sites. Image used with permission from sunraisedfarms.com.

Based on the U.S. Department of Energy’s SunShot Vision Study, solar energy capacity in the year 2030 could reach 329 gigawatts (GW), which would
require approximately 1.8 million acres of land. While this represents less than 0.1% of total land area in the contiguous United
States, there is growing concern over the land use impacts of solar energy development
on fertile agricultural land, which can consequently displace farming and food production.
As a result, states have started to enact policies aimed at protecting agricultural
land from solar development. For instance, Senate Bill No. 943 (which restricts PV facilities of over 2 megawatts on farmland) recently passed the
Connecticut General Assembly and counties in North Carolina and Washington have imposed restrictions on large scale solar projects citing concerns over the
loss of farmland.

As an alternative to restricting solar installations on agricultural land via policy
measures, low-impact solar development and co-location of solar and agriculture is a technique currently being researched
that has the potential to alleviate agriculture displacement by allowing solar arrays,
vegetation, and livestock to occupy the same land area.

Low-impact solar development can serve a variety of project goals and can be divided
into three categories:

Solar-Centric Design

Optimized for solar output but keeps low-lying vegetation for ground cover and habitat.

Vegetation-Centric Design

Optimized for vegetation growth but allows for solar arrays to be located in areas
where vegetation will not be negatively affected (see Figure 2).

Co-Location Design

Solar and vegetation configurations are designed jointly for maximum dual output.

All three of these categories can include leaving existing vegetation in place or
replacing it with low-growing native vegetation, designing the solar project around
natural land contours, and having vegetation that support habitats (for example, pollinator
species or livestock). Planning and implementing this vegetation co-location during
the initial site preparation can help lower the overall installed cost by an estimated 3%-8% per watt compared to common industry practices in solar development, with cost reductions
expected in the following site preparation categories:

When farms use center-pivoted irrigation, the outer corners of square plots of land
(shown in yellow) are not used for crop growth and could be utilized for a veggie-centric
solar installation. Source: https://www.nrel.gov/docs/fy11osti/51330.pdf.

One concern with growing food crops or natural vegetation under solar modules is the
effects of shading and rain run-off. Solar panels cause ground shading throughout
the day (although the amount of shading varies based on if the land parcel is directly
beneath a panel or between rows) and precipitation tends to drip off the side of the
panel that is tilted closer to the ground, both of which can affect plant growth.
The NREL technical report Native Vegetation Performance under a Solar PV Array at the National Wind Technology
Center systematically studied to what extent plant cover can be re-established and thrive
under solar arrays, ultimately concluding that certain types of vegetation can produce
an extensive plant cover within a 3-year period. While this study was location specific,
the observed effects of shading and general plant growth may be widely applicable
to a variety of sites and climates.

An additional solar and agriculture co-location technique is to keep a herd of sheep
or other livestock within the enclosure of a ground-mounted PV system to help control
vegetation height. Doing so can significantly reduce operation costs for solar developers.
A spokesperson for Duke Energy, one of the largest electric utility companies in the United States,
noted that apart from the lease of the land, vegetation management (i.e., lawnmowing)
is the primary operating expense at their solar facilities. A mutually beneficial
partnership can be established between sheep farmers and solar developers, where the
developers provide grazing land at the site of their solar development and the sheep
provide low-cost vegetation control. Multiple sheep farming companies have been founded
on this principle and can help provide a key source of revenue for 21st-century farmers. Other types of animals have been tested for solar vegetation management, but sheep have often proven to be the best tenants of the land. Horses can be picky about what they eat, cows
are large and require a lot of space, and goats tend to chew on wires and climb on
panels.

Co-location of solar and agriculture can have multiple benefits for both land owners
and solar developers, as highlighted in the table below.

Benefits to Land Owners

Benefits to Solar Developers

Self-generation of electricity and reduced energy bills

Reductions in site preparation and installation costs

Control of wind and soil erosion

Reductions in operation and maintenance costs

Compatible with grazing activities, provides shade and cover for livestock